The present invention relates to a solid-state sensor device for determining the concentration of gas species, in particular oxygen, which sensor device is capable of operating also at room temperature.
The determination of oxygen content in a gas mixture, or in liquids, is a very important problem, from the industrial, biological, environmental, and still other, viewpoints.
Very often, a continuous monitoring is required, as in the case of rivers, lakes, or seas, in order to keep controlled the level of pollution by organic materials. Such a need may arise also in the case of biological liquids in general, or when dealing with gases generated by combustion processes, and so forth.
For such purposes, sensor devices of voltammetric type are normally used [see: L. C. Clark, Jr., Trans. Am. Soc. Artif. Intern. Organs 2, 41 (1956)].
During the past ten years, also potentiometric systems were introduced, which make use of a solid O.sub.2 conductor, usually yttrium-doped zirconia. Unfortunately, all such potentiometric systems have high operating temperatures, higher than 300.degree.-400.degree. C. [see: E. Siebert, J. Fouletier, S. Vilminot "Solid State Ionics", 9& 10 (1983) 1291].
Very recently, attempts were carried out aiming at decreasing the operating temperatures of the potentiometric sensor devices, by studying different types of solid electrolytes [see: Miura, J. Hisamoto. S. Kuwata, N. Yamazoe, "Chemistry Letters" (1987) 1477; T. Inoue, K. Eguchi, H. Arai "Chemistry Letters" (1988), 1939; S. Kuwata, N. Miura, Y. Yamazoe, "Chemistry Letters" (1988), 1197].
Recently, the same Applicant developed a potentiometric sensor device for oxygen, operating at room temperature, claimed in European Patent Application Public. No. 0 432 840, in order to determine the concentration of gases which can react with hydrogen, in particular oxygen, which sensor device is essentially constituted by a solid-state protonic conductor kept into contact, at one of its sides, with a reference electrode--constituted, in its turn, by a hydride of a metal or metal alloy--and, at its other side, with an electrode which catalyzes the reaction of the gas to be detected, with hydrogen.
Such a sensor device is based on the measurement of the mixed potential which is generated on the catalytic electrode in the presence of oxygen and hydrogen.
In such a sensor device, hydrogen is produced by current or potentiostatic impulses, and the potential is measured after a suitable time interval between each impulse and the subsequent impulse.
The sensor device according to the above cited patent application resulted to respond to up to a few parts of oxygen per million parts (ppm).